Analgesic agent and its use

Abstract: Levobupivacaine is useful as an anaesthetic in obstetrics.

Description: Field of the Invention This invention relates to a new therapeutic use for levobupivacaine, i.e. the (S)-enantiomer of 1-butyl-N-(2,6-dimethylphenyl)-2-piperidinecarboxamide. Background of the Invention Racemic bupivacaine is an effective long-acting local anaesthetic, and may be given as an epidural. However, racemic bupivacaine is cardiotoxic, having depressant electrophysiological and mechanical effects on the heart. It should therefore be used with caution in cardiac-compromised patients. It is known that levobupivacaine is probably less cardiotoxic than dexbupivacaine and racemic bupivacaine. See, for example, Vanhoutte et al , Br. J. Pharmacol. 103 :1275-1281 (1991), and Denson et al , Regional Anaesthesia, 17 :311-316 (1992). Vanhoutte et al studied the effects of bupivacaine enantiomers on the electrophysiological properties of guinea pig isolated papillary muscle; this is based on their statement that "the cardiotoxicity of bupivacaine seems to be mainly of electrophysiological origin". Mazoit et al , Anesth. Anal. 77 (3) :477-482 (Sept. 1993), disclose that (S)-bupivacaine exerts less detrimental effect on the rabbit heart than the (R)-enantiomer or the racemate. In referring to the prior art, the (S) -enantiomer is reported to have a longer duration of action than the (R)-enantiomer. US-A-4695576 discloses that research on the optical isomers of bupivacaine revealed that the differences in potency compared to the racemate were not of such a magnitude as to justify commercial production of the optical isomers. Data presented for the duration of analgesia demonstrate that each isomer has less activity than the racemate. Kuhnert et al , Federation Proceedings 40 (31):684(1981), discusses bupivacaine disposition in mother, foetus and neonate. It is reported that the low foetal/maternal ratio of bupivacaine at birth is due to considerable uptake of bupivacaine by foetal tissues and is not due to diminished placental transfer. Summary of the Invention According to the present invention, levobupivacaine or a salt thereof is used for the manufacture of a medicament for use as an anaesthetic in obstetrics. In particular, it has now been found that levobupivacaine in man has less effect on stroke, cardiac and acceleration indices than racemic bupivacaine. As these are indices of myocardial contractility, this finding indicates that levobupivacaine is less cardio-depressant than racemic bupivacaine. This finding supports the use of levobupivacaine as a safer long-acting local anaesthetic for use in patients having or disposed to depressed myocardial contractility, i.e. a class of cardiac-compromised patients and also patients undergoing concomitant therapy with medicines having a cardio-depressant effect or with cytotoxic drugs. For the same reasons, levobupivacaine may also have beneficial therapeutic effects after certain kinds of surgery when sympathetic blockade is required with minimal cardio-depressant effects. The agent may be the single isomer, but is effectively free of dexbupivacaine, i.e. in at least 80%, more preferably at least 90%, and most preferably at least 99%, enantiomeric excess. Any conventional salt, e.g. the hydrochloride, may be used. Description of the Invention The concentration of levobupivacaine to be given for effective utility, is for example, 0.25%, 0.5% or 0.75%, depending on the procedure envisaged. Up to 60 ml in a single dose can be given. The usual routes of administration are infiltration, epidural, spinal and peripheral nerve block. It is also possible to provide continuous infusion of agent at lower concentration, for example 0.125%, with or without opioid, depending on anaesthetic practice. This is common during labour and is distinct from the treatment of chronic pain when infusions can continue for days rather than hours. An additional benefit of levobupivacaine over racemic bupivacaine is its reduced uptake into heart and brain. It is therefore particularly suitable for use in treating chronic pain. This is described more fully in WO-A-9 510 276. The following provides the evidence that is the basis of the present invention. The cardiovascular and central nervous effects of levobupivacaine were compared with racemate (Marcaine) in healthy male volunteers. Drugs were administered by intravenous administration in a double-blind crossover manner. The infusion rate was 10 mg/min for each drug and infusion was continued up to a maximum of 150 mg or stopped following the first detection of CNS effects (including tinnitus, numb tongue or lips and dry mouth). Volunteers were previously conditioned to the CNS effects of local anaesthetics by administration of a test dose of lignocaine. A range of cardiovascular parameters were measured, including systolic and diastolic blood pressures, ECG, and, using the transthoracic electrical bioimpedance technique (with a BoMed NCCOM3-R7), aortic blood flow, allowing measurements of cardiac index and stroke index. Based on results from a previous study in which racemic bupivacaine was infused it was anticipated that the major cardiovascular changes observed following bupivacaine administration would be related to myocardial contractility. Therefore, an acceleration index, representing the initial maximum acceleration of blood flow during the onset of ejection, was measured to estimate myocardial contractility in this new study. Levobupivacaine, like racemate, was well tolerated. The mean total doses administered of levobupivacaine and racemate were 54.0 and 45.6 mg respectively and plasma concentrations at the end of infusion were 2.384 µg/ml and 1.87 µg/ml respectively (n=12). Despite the mean total dose and plasma concentration being higher with levobupivacaine, this produced much smaller mean changes in cardiac variables than the racemate. The myocardial contractility index was significantly reduced by bupivacaine from a value of 1.36 s -2 to 1.18 s -2 , a decrease of 0.18 s -2 or 13%. For levobupivacaine the pre-dose value was 1.34 s -2 and this only decreased to 1.28 s -2 at the end of infusion, a decrease of 0.06 s -2 or 4.5%. The difference between drug treatments was highly significant (p <0.02, n=12). The results were similar for stroke index, a parameter likely to reflect changes in myocardial contractility. Bupivacaine reduced this from 55.3 ml/M 2 to 44.4 ml/M 2 , a decrease of 10.9 ml/M 2 or 20%. For levobupivacaine the pre-dose value was 52.4 ml/M 2 and 49.1 ml/M 2 at the end of infusion, a decrease of 3.3 ml/M 2 or 6%. Again the difference between drug treatments was statistically highly significant (p <0.01, n=12). Small changes in other variables occurred including heart rate and mean blood pressure (increases) and ejection fraction and cardiac index (decreases). As with acceleration index and stroke index the changes tended to be greater with bupivacaine. A second study has been carried out, to compare the efficacy of levobupivacaine against bupivacaine using the ulnar block technique. Concentrations of 0.125%, 0.25% and 0.5% levobupivacaine were compared with 0.25% bupivacaine (all volumes 5 mls) in a double-blind study in 20 volunteers. A preliminary analysis of the data suggests that in terms of sensory block, levobupivacaine has comparable efficacy to bupivacaine, with the duration of sensory block to 0.25% bupivacaine being similar to that seen with levobupivacaine 0.25%. These results with levobupivacaine have provided evidence that this local anaesthetic, in comparison with the currently clinically-used racemate, has a lower potential to cause cardiotoxicity in man. This, along with evidence that levobupivacaine has a similar anaesthetic potency to the racemate, suggests that levobupivacaine will be a safer local anaesthetic in the clinic. This would be of particular importance for obstetrics use. Based on preclinical results obtained with separated enantiomers, the reduced cardiotoxicity of levobupivacaine in man is likely due to reduced direct actions on the heart. However additional factors may contribute to the reduced cardiotoxicity. Recently, others have reported stereoselective plasma binding of bupivacaine enantiomers in man, with the plasma binding of levobupivacaine being about 50% higher than binding for dexbupivacaine. This higher binding would reduce the consequences of accidental intravascular administration.

Claims: Use of levobupivacaine or a salt thereof, for the manufacture of a medicament for use as an anaesthetic in obstetrics. Use according to claim 1, wherein the levobupivacaine is in at least 90% enantiomeric excess with respect to dexbupivacaine.

Inventors: BARDSLEY HAZEL, GRISTWOOD ROBERT WILLIAM, BARDSLEY, HAZEL, GRISTWOOD, ROBERT WILLIAM
Applicants: DARWIN DISCOVERY LTD, DARWIN DISCOVERY LIMITED
Application year: 1994
Main classification: A61K31
IPC codes: A61K31